Targeting Lag-3 and PD -1 in Myeloid Cells of GBM

Glioblastoma (GBM) is the most aggressive type of primary malignant brain tumor in adults. GBM has a bleak prognosis of approximately 12-15 months, despite continuing advances to the standard of care. Immunotherapy has demonstrated the significant potential to boost immune responses against many cancer types; inhibitors of checkpoint molecules, such as CTLA-4 and PD-1, have been used to treat many solid tumors with varying success. While it has demonstrated efficacy in treating some tumors with an inflammatory milieu and high degree of infiltration by anti-tumor T cells, it has shown little to no response in treating tumors such as GBM. This tumor type is characterized by a particularly immunosuppressive microenvironment with a notable paucity of T cells. Targeted approaches designed to reduce immunosuppression in the tumor and increase anti-tumor T cell activity are crucial to successfully treat GBM. Recent preclinical data from our laboratory and preliminary findings from a Phase I clinical trial have shown promising signs of efficacy with co-blockade of PD-1 and the alternative checkpoint LAG-3. We have observed long-term survivors and radiographic responses in trial patients. We have noted improved T cell responses against the tumor and a reduction in myeloid-derived suppressor cells (MDSCs), following dual therapy of patients. We propose to study the mechanism by which the immune system is enhanced against GBM via PD-1/LAG-3 blockade. We hypothesize that dual therapy recruits cells of the myeloid compartment in order to boost anti-cancer T cell activity, and reduces the presence of immunosuppressive myeloid cells in the tumor itself. To test our hypothesis, we will investigate the: i) priming of T cells by antigen presenting myeloid cells in response to dual therapy in murine models of GBM, ii) role of soluble product generated from cleavage of surface LAG-3 molecule in inducing myeloid-mediated immunosuppression in the tumor (using both murine models and patient samples), and iii) expansion of anti-tumor T cells and reduction in MDSCs levels in patients following dual therapy. We will correlate our findings with overall survival and progression free survival of the trial patients. We expect that the data generated from these studies will provide novel insights into a previously unexplored mechanism by which dual immune checkpoint blockade therapy can modulate the myeloid response against tumors. The knowledge obtained from this study will contribute to improving the design of future therapeutic strategies to treat GBM patients.